Corynebacterium Diphtheriae: Infection, Transmission & Diseases

Corynebacterium diphtheriae is a gram-positive rod bacterium that belongs to the genus Corynebacteria. It causes the disease diphtheria.

What is corynebacterium diphtheriae?

Corynebacteria belong to the gram-positive rod bacteria. Gram-positive bacteria can be stained blue in the Gram stain. Unlike Gram-negative bacteria, they possess only a thick peptidoglycan layer of murein and have no additional outer cell wall. Corynebacteria are immobile and cannot form spores. Due to their swollen cell ends, rod-shaped bacteria have the shape of a club. They have the ability to grow under both anaerobic and aerobic conditions. Corynebacterium diphtheriae has a diameter of 0.5 micrometers. It is between two and four micrometers long. Characteristic of this bacterial strain is the grouped arrangement, which resembles a V. A total of four different biotypes can be distinguished. The gravis, belfanti, mitis, and intermedius types differ in terms of sugar fermentation reactions, hemolytic activity, and in terms of their colonization formation.

Occurrence, distribution, and characteristics

Infections with Corynebacterium diphtheriae occur worldwide. Most disease is observed in temperate climates. Infections occur more frequently in autumn and winter. Over the past 50 to 70 years, a sharp decline in Corynebacterium diphtheriae infections has been observed in western industrialized countries. However, diphtheria still occurs endemically in other parts of the world. Endemic areas include Afghanistan, Indonesia, India, Haiti, some African countries, and Russia. The last major German epidemic with Corynebacterium diphtheriae was in the years 1942 to 1945, and only isolated cases of infection have been documented since 1984. For Corynebacterium diphtheriae, humans are the only relevant reservoir. Transmission occurs when the throat is infected by droplet infection. This transmission variant is also called face-to-face contact. In the case of cutaneous diphtheria, infection occurs through direct contact. Asymptomatic carriers, so-called excretors, transmit the pathogen less frequently than persons who are actually ill. For every 100 people exposed to the pathogen, about 10 to 20 become ill. This corresponds to a contact index of 0.1 to 0.2. The contact index describes the proportion of the non-immune population in which infection occurs after contact with the respective pathogen of the disease. Although infection through contact with contaminated material is theoretically possible, it occurs rather rarely. Infections can also occur occupationally in the laboratory. However, the last reported laboratory infection with Corynebacterium diphtheriae occurred in the 1990s. The incubation period for infection with Corynebacterium diphtheriae is two to five days. In rare cases, the first symptoms do not appear until after eight days. Contagiousness persists as long as the pathogen is detectable. Without treatment, most patients are contagious for about two weeks. Rarely, contagion still occurs after more than four weeks. When treated with antibiotics, contagiousness persists for only two to four days.

Diseases and symptoms

Corynebacterium diphtheriae can cause diphtheria only if it can produce diphtheria toxins. The exotoxin is produced only when the bacterium is infected by a bacteriophage. Bacteriophages are virus species that specialize in infecting bacteria. Infections with Corynebacterium diphtheriae in temperate climates mainly affect the respiratory tract. Primary infection occurs primarily in the tonsils and throat. However, primary infection of the larynx, nose, trachea, or bronchi may also be present. Diphtheria usually begins with a sore throat and difficulty swallowing. Symptoms are accompanied by fever up to 39°C. Later, patients suffer from hoarseness and swelling of the lymph nodes. A gray-white coating forms on the tonsils and in the throat. The coating may also appear brownish and is called a pseudomembrane. Often, this pseudomembrane exceeds the tonsils and spreads to the area of the palate and on the uvula.Attempts to lift off the membrane with a wooden spatula result in punctate hemorrhages. These punctate hemorrhages are an important diagnostic criterion for distinguishing diphtheria from other diseases of the respiratory tract. A sweetish odor is also typical of diphtheria. It can be perceived even at some distance. Massive swellings occur in the area of the throat. Due to them, the characteristic image of Caesar’s neck is formed. The swelling can be so severe that it causes obstruction of the airways. In particular, in laryngeal diphtheria, called true croup, choking can occur. Other symptoms of laryngeal diphtheria include cough and hoarseness. Nasal diphtheria is much less noticeable. Here, only a slightly bloody discharge from one or both nostrils is often seen. The most significant complications of diphtheria are choking, inflammation of the heart muscle, and nerve inflammation. Such polyneuritis may occur for weeks after the actual disease. Rarer complications include kidney failure, cerebral infarction, encephalitis, or pulmonary embolism. Cutaneous or wound diphtheria occurs predominantly in tropical areas. In Western countries, risk groups such as the homeless or drug addicts are affected. Based on the clinical picture, a skin infection with Corynebacterium diphtheriae cannot be distinguished from other bacterial skin infections. Five to ten percent of all diphtheria patients die despite treatment. If treatment is delayed or medical care is inadequate, lethality increases to as much as 25 percent.